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Java – Object & Classes

Java is an Object-Oriented Language. As a language that has the Object Oriented feature, Java supports the following fundamental concepts:

  • Polymorphism
  • Inheritance
  • Encapsulation
  • Abstraction
  • Classes
  • Objects
  • Instance
  • Method
  • Message Parsing

In this chapter, we will look into the concepts Classes and Objects.

Object – Objects have states and behaviors. Example: A dog has states – color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.

Class – A class can be defined as a template/blue print that describes the behaviors/states that object of its type support.

Object in Java

An entity that has state and behavior is known as an object e.g. chair, bike, marker, pen, table, car etc. It can be physical or logical (tengible and intengible). The example of integible object is banking system.

An object has three characteristics:

state: represents data (value) of an object.
behavior: represents the behavior (functionality) of an object such as deposit, withdraw etc.
identity: Object identity is typically implemented via a unique ID. The value of the ID is not visible to the external user. But,it is used internally by the JVM to identify each object uniquely.
For Example: Pen is an object. Its name is Reynolds, color is white etc. known as its state. It is used to write, so writing is its behavior.
Object is an instance of a class. Class is a template or blueprint from which objects are created. So object is the instance(result) of a class.

Class in Java

A class is a group of objects that has common properties. It is a template or blueprint from which objects are created.
A class in java can contain:

  • data member
  • method
  • constructor
  • block
  • class and interface

Syntax to declare a class:

class <class_name>{
data member;
method;
}

A class can contain any of the following variable types.

Local variables: Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed.

Instance variables: Instance variables are variables within a class but outside any method. These variables are initialized when the class is instantiated. Instance variables can be accessed from inside any method, constructor or blocks of that particular class.

Class variables: Class variables are variables declared with in a class, outside any method, with the static keyword.

Example of Object and Class

There is given another example that maintains the records of Rectangle class. Its exaplanation is same as in the above Student class example.
class Rectangle{
int length;
int width;

void insert(int l,int w){
length=l;
width=w;
}

void calculateArea(){System.out.println(length*width);}

public static void main(String args[]){
Rectangle r1=new Rectangle();
Rectangle r2=new Rectangle();

r1.insert(11,5);
r2.insert(3,15);

r1.calculateArea();
r2.calculateArea();
}
}
Output: 55
45

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